The invention relates generally to state lasers and more particularly to chromium doped solid state lasers.
One concept for a 10 MJ fusion research laser uses a gain medium that is pumped to a high stored energy density (greater than 2 J/cc) by a long pulse (greater than 1 msec), efficient (greater than 10%), low cost (less than 1$/cc) laser operating near 800 nm. A phonon terminated chromium doped solid state laser is one possible type of pump laser. Calculational modelling of such lasers advises that ten percent efficient chromium lasers are a reasonable technical goal. The alexandrite chromium laser has the highest known performance (5% efficient, projected cost 2-4$/cc); it is believed to be limited in efficiency to 5% by small, but significant excited state absorption (ESA) losses. Accordingly, it would be desirable to find a 10% efficiency chromium laser (with reduced ESA and other losses) and that otherwise has the potential to meet the cost goal.
The detrimental effects of excited state absorption (ESA) have been investigated for the case of solid state lasers based on the d.sup.3 impurities, Cr.sup.3+ and V.sup.2+. It was shown that the infrared ESA band overlaps the emission band, thereby reducing the efficiency of the Na.sub.3 Ga.sub.2 Li.sub.3 F.sub.12 :Cr.sup.3+ and KMgF.sub.3 :V.sup.2+ lasers. A fundamental understanding of this ESA band was devised in terms of lattice relaxation around the impurity site and its consequences. Within the context of this model, it was determined that the best fluoride laser hosts for Cr.sup.3+ would involve: (a) the crystal sites affording the highest possible crystal field strength commensurate with .sup.4 T.sub.2 -.sup.4 A.sub.2 emission, and (b) small substitutional sites in lattices with the greatest amount of stiffness. These criteria are compatible with the materials that are expected to have the best thermo-mechanical properties. Additionally, it appears that crystals that offer only a single type of Cr.sup.3+ site are "safer", as the chance of one of the sites having unsuitable spectroscopic properties is substantial. Thus, it would be desirable to find a fluoride crystal which satisfied all of these requirements.
One particular fluoride crystal, colquiriite (LiCaAlF.sub.6), has been previously considered as a possible Nd.sup.3+, Cr.sup.3+ co-doping candidate, in which the Cr ion is used for sensitization of the Nd laser. As a sensitizer the Cr ion absorbs some of the pump energy and transfers energy to the Nd ion to excite the Nd ion to produce laser action of the Nd ion. Colquiriite is known to have good Cr.sup.3+ fluorescence properties and to grow in single crystal form.